a) Field of the Invention
The present invention relates to thermal treatment, and more particularly to thermal treatment for the oxidation, diffusion, and the like to be performed on semiconductor wafers.
b) Description of the Related Art
A vertical type thermal treatment furnace such as shown in FIGS. 3 and 4 is known in which semiconductor wafers are subjected to an oxidation process, an impurity diffusion process, or the like.
In FIGS. 3 and 4, reference numeral 10 represents a process tube disposed upright which is made of, for example, a circular quartz tube. Although the top of the process tube 10 is closed, the top is shown broken in FIG. 3 so as to expose the interior of the process tube
Two gas supply pipes 12 made of quartz and branched from a lower gas supply pipe 12o extend upward along the side wall of the process tube 10. A processing gas is introduced into the upper space of the process tube 10 via the gas supply pipes 12o and 12. By blowing out the processing gas in two directions, the uniformity of the gas in the process tube 10 is improved. As the processing gas to be used for an oxidation process, an oxidizing gas (a reaction product of H.sub.2 and O.sub.2, containing oxygen and water vapor) generated at an external gas burner 14 is used.
The gas introduced into the process tube 10 is exhausted from a gas exhaust pipe 16 provided at the lower portion of the process tube 10 to an exhaust system. As shown in FIG. 4, a heater 18 is mounted surrounding the gas supply pipes 12 and process tube 10.
In operation, semiconductor wafers WF supported by a wafer holder (not shown) are transported into the process tube from the bottom thereof. The bottom of the tube 10 is closed by a lid formed integrally with the wafer holder.
With the vertical type heat treatment furnace described above, however, an oxide film formed on a wafer area S (shown in FIG. 4) near the gas supply pipes 12 becomes thinner and an impurity concentration of a film near the gas supply pipes 12 becomes lower, because of the heat capacity of the gas supply pipes 12. For example, an experimental oxidation process was performed to form a thermal oxide film having a thickness of 15 nm on the surface of a silicon wafer. The film thickness at the wafer area S was thinner than the other area by 2 to 3 %.
Another problem is the long time required for processing gas replacement, or in other words, a long gas stay time in the process tube, because the processing gas is introduced into the upper space of the process tube 10 and then exhausted from the lower portion thereof.
An increased gas flow rate has been proposed the shorten the gas replacement time. However, this method poses the problem of an increased variation of oxide film thickness in the wafer plane at the lower portion of the furnace, as shown in FIG. 5 which shows the relationship between an H2 flow rate on the abscissa and a variation of oxide film thickness in a wafer plane on the ordinate. In the experiment shown in FIG. 5, mixing rates of H.sub.2 and O.sub.2 are kept constant. Curves SB, SC, and ST represent film thickness variations at the lower, middle, and upper portions of the furnace, respectively.
As seen from the graph of FIG. 5, the oxide film thickness variation is larger at the furnace lower portion than at the furnace middle and upper portions. This means that the reaction conditions (water vapor concentration, reaction time, and the like) change with the position in the furnace and that there is an intra-batch process variation. There is also a large inter-batch process variation because a gas replacement is performed at each oxidation process.